Entropy & Feasibility

• Entropy (∆S)


  A measure of disorder, the more disordered a substance, the greater its entropy value
• As temperature increases


  Entropy increases because the particles gain energy and move further apart, becoming less ordered

  
• Entropy of different states of matter

  • Gases have the greatest entropy
  • Solids and liquids have lower entropy

  
• When a substance melts or evaporates

  1. There is a sudden increase in entropy

  2. The entropy change of vaporisation is much greater than that of fusion as a gas is much more disordered than a liquid or a solid

  
• Overall entropy change (∆S) for a reaction

  • Measured in JK-1mol-1
  • All spontaneous reactions have a positive entropy value, ie. disorder increases

  
• Gibbs Free-Energy (∆G)


  • Allows the entropy change of a reaction to be found without needing to measure the effects on the surroundings
  • Uses the reaction temperature, enthalpy (∆H) and entropy (∆S) changes to determine a value for ∆G

  
• ∆G is measured in kJmol-1 so it is important that ∆H and ∆S are used in the same, standard units
• ∆G against ∆S


  Can be represented graphically as a straight line graph
• ∆G


  • Negative for all spontaneous reactions
  • Becomes zero at a certain, specific temperature
  • Changing the temperature or the type of reaction occurring will change the feasibility and spontaneity of the reaction
• Relationship between ∆H, ∆S and feasibility

  • ∆H positive, ∆S positive, T∆S > ∆H: Feasible above a certain T value
  • ∆H positive, ∆S positive: ∆G gets more negative, Always feasible
  • ∆H positive, ∆S negative: ∆G always remains positive, Never feasible
  • ∆H negative, ∆S negative: Usually ∆H > T∆S, Usually feasible